The PRC was definately a marketing move by Hornady, and in my mind maybe not as well thought out as it shoulda been. Re: we see a lot guys now building off of long actions, where the whole point was to stay in SA mag length. I've been a 6.5 RSAUM shooter in its GAP 4S version a while and its certainly capable of killing elk, I certainly prefer a bit more horsepower at the longer ranges we see guys shooting today.
The bullet technology is certainly available when going into "tuff" category in the 6.5mm but your going to be pushing short action length constraints. To optimize those heavier bc's you best to look at Sherman's and or the 6.5-XX wildcats, I shoot a 6.5-280 AI also. But believe it or not my truck gun is a short barrel 6.5 Creed. (But I think I'm going to build a 6 Creed to get bit more velocity and less drop for those quick snap shots that happen a lot on those pesky coyotes!).
But back to the basis of your question, definitely the 6.5 PRC is waining a bit, the PRC crowd has moved onto the 6's n the 25's and even now the 22 CM, XC, BR, n XX-47's taking a lot of the thunder of why the PRC was developed.
We'll see how much enthusiasm there is similarly for the 300 PRC and over the years how much it wains, but in my opinion the 300PRC fills a better void of "efficiency" in large 30's better than others and 210's n 230's kill elk with a hell of a lot more authority than any 6.5!!